Automobiles, etc. equipped with an internal combustion engine (hereinafter referred to simply as “engine”), such as a diesel engine or a gasoline engine, have a catalytic converter for cleaning up exhaust gas of the engine, and such catalytic converter contains catalysts, such as three-way catalysts. Such catalytic converter is not activated, thus does not exhibit its capacity to promote reduction or oxidation of exhaust gas unless it undergoes a temperature rise. Thus, temperature control over the catalytic converter is important.
Immediately after the engine is started, however, the temperature of the exhaust gas is low, so that it takes a certain time (warming-up time) for the catalytic converter to become activated, or in other words, rise to its activation temperature. In order to alleviate this problem, a measure to cause early activation of the catalytic converter is taken. Such measure is, for example heating the catalytic converter by a heater or increasing the idling speed of the engine immediately after starting the engine.
There are known, on the other hand, engines provided with a supercharger, in which the supercharger is used to increase the power output of the engine. A turbocharger, which is a type of supercharger, comprises a turbine rotated by kinetic energy of exhaust gas and a compressor driven by the turbine to compress air flowing toward the engine and deliver the compressed air to the engine.
In the engine provided with a turbocharger, the catalytic converter is arranged normally downstream of the turbocharger in the exhaust passage of the engine, so that the turbocharger absorbs thermal energy of exhaust gas according to its heat capacity, and reduces heat supplied to the catalytic converter. Consequently, engines with a turbocharger take longer time to warm up the catalytic converter, compared with engines without a turbocharger. Thus, in the engines with a turbocharger, how short the catalytic converter warming-up time can be made is important.
For example, Patent Document 1 discloses a turbocharger control device, which determines whether or not the catalytic converter needs warming, and if the catalytic converter needs warming, controls the revolving speed of the turbine, thereby reducing a difference between the pressure of exhaust gas at the inlet of the turbine, which will be referred to as “inlet pressure”, and the pressure of exhaust gas at the outlet of the turbine, which will be referred to as “outlet pressure”.
The above control device can reduce the difference between the inlet pressure and the outlet pressure to a level as close to zero as possible in a short time. Specifically, in the case that the turbocharger is a variable displacement turbocharger with a variable nozzle, the control device opens the vane of the variable nozzle to its maximum extent, thereby greatly reducing the difference between the inlet pressure and the outlet pressure. This results in a reduction in the velocity of exhaust gas flowing through the turbine, and therefore, a reduction in the thermal energy of exhaust gas consumed to rotate the turbine. This allows the catalytic converter to be efficiently warmed up in a short time.